1. Field of the Invention
The present invention relates to modulating signals to be transmitted in communication systems. Although useful for many types of communication systems, the present invention, a quadriphase-shift keying modulator, is particularly useful for a satellite communication system and is described in that connection.
2. Description of Related Art
In a typical satellite communication system, outgoing RF (radio frequency) signals transmitted from a mobile terminal unit are received directly by the satellite. The satellite in turn retransmits the RF signals to a ground station that is connected by wire to a public switched telephone network (PSTN), which in turn routes the outgoing signals to either a conventional telephone or to another mobile terminal unit of a satellite or cellular network. Incoming signals from a conventional wired telephone are conducted from the PSTN to the satellite ground station, which in turn transmits RF signals to the satellite for retransmission to the mobile terminal unit. Thus, communication can occur between two mobile terminal units or between a mobile terminal unit and a conventional telephone connected to a PSTN, or between a satellite mobile terminal unit and a cellular mobile terminal unit, for example. In each of the aforesaid conditions, the communication is routed through a PSTN.
It has been proposed to provide a satellite communications network that utilizes high altitude geosynchronous satellites each of which is capable of covering an area corresponding to a substantial portion of the North American continent, so that a single satellite with approximately six beams will cover the entire continent from Alaska to Mexico. The satellite for such a network will be approximately 22,600 miles above the equator and will be designed to operate in the L-Band of RF frequencies. For example, the frequency of the signal being transmitted to the satellite will be between 1626 MHz to 1660 MHz; and the frequency of the signal received from the satellite will be between 1525 MHz to 1559 MHz.
In such a satellite communications network, the information is processed in digital form prior to transmission, and the digital information is modulated onto a carrier frequency before being transmitted.
Common forms of modulation include frequency modulation, amplitude modulation, and phase modulation.
In typical forms of phase modulation, pairs of digital bits comprising symbols which represent four different possibilities of data to be modulated onto the carrier wave are input to an analog baseband waveform generator. The baseband waveform generator produces an inphase waveform and a quadrature waveform. These two waveforms are then used to drive a single sideband modulator, or complex up-converter, to produce an Intermediate Frequency (IF) or RF waveform.
The typical direct up-converters and modulators require dual digital-to-analog (D/A) converters for the inphase and quadrature components. The D/A converters introduce imbalances between the inphase and quadrature components due to manufacturing tolerances and temperature differences in the D/A converter components and imprecise gains in the subsequent amplifiers. Furthermore, the typical digital system does not lend itself to efficient implementation.
In light of the foregoing, there is a need for a terminal unit that is not only capable of reliably modulating digital signals onto to a carrier frequency for voice, facsimile, and data communication, but which is also compact, lightweight, and relatively inexpensive to manufacture.